Two pink four-o'clock flowers are crossed and give red, pink and white in a 1:2:1 ratio, not 3:1. Why?
Flower colour here shows incomplete dominance, so the heterozygote (Rr) is pink -- a visible blend, not red.
Flower colour here shows incomplete dominance, so the heterozygote (Rr) is pink -- a visible blend, not red. Crossing pink x pink (Rr x Rr) gives 1 RR (red) : 2 Rr (pink) : 1 rr (white). Because the heterozygote looks different from either homozygote, the phenotype ratio matches the genotype ratio (1:2:1) instead of collapsing into the usual 3:1 of complete dominance.
Key point
Incomplete dominance makes the heterozygote distinct, so phenotype ratio equals genotype ratio, 1:2:1.
Common mistake
expecting a 3:1 ratio whenever two hybrids are crossed.
Memory tip
when the hybrid has its own appearance, phenotype ratio = genotype ratio (1:2:1).
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